Thin-film transistors (TFTs) are used in the construction of active-matrix liquid-crystal displays, active-matrix organic light emitting diode displays, active-matrix e-ink electronic books and active-matrix image sensors. TFTs based on amorphous silicon suffer from low operating speed and lack of a p-type device, making it difficult to realize peripheral circuits. There has therefore been a move to polycrystalline silicon devices, which can be obtained from amorphous silicon by furnace or laser-induced heating.
Among the important requirements for the realization of thin-film semiconductor devices are (a) quality, (b) uniformity and reproducibility, (c) maximum fabrication process temperature and (4) manufacturing cost. Polycrystalline silicon can be obtained by metal-induced crystallization of amorphous silicon at a temperature below 550° C. using low-cost conventional furnaces. The resulting material contains large grains with continuous grain boundaries, and uniform and reproducible material and device characteristics. However, better material and device performance is possible if material micro-defects can be reduced.
Excimer laser crystallization of amorphous silicon can be utilized to produce good quality polycrystalline silicon. However, the quality is not uniform and suffers from poor reproducibility. The cost is also high.
Polycrystalline silicon obtained by conventional low-pressure chemical vapor deposition suffers from high process temperature (620-650° C.) and poor material quality. The quality can be improved using high temperature annealing (above 900° C.). However, this is only possible with the much more expensive quartz substrates and not with inexpensive glass substrates.
An alternative technique of obtaining polycrystalline silicon is metal-induced crystallization (U.S. Pat. Nos. 5,275,851; 5,879,977; 6,737,674; and U.S. Patent Publication 2001/018224). The resulting material contains large grains with continuous grain boundaries, and uniform and reproducible material and device characteristics. However, improved material and device performance is still possible if material micro-defects can be reduced.
Another technique is excimer laser crystallization of amorphous silicon, which can be utilized to produce good quality polycrystalline silicon (U.S. Pat. Nos. 5,352,291; 6,071,796; and U.S. Patent Publication 2004/087116). However, the resulting quality is not uniform and suffers from poor reproducibility. The cost is also high.
Combining metal-induced crystallization and excimer laser annealing has been proposed in U.S. Pat. Nos. 5,705,829; 5,893,730; 5,869,362; and U.S. Patent Publication 2003/129853). However, the excimer laser is expensive and complex. Polycrystalline silicon can be obtained using Nd:YAG laser heated conductor layer on amorphous silicon (U.S. Pat. No. 6,537,864). While the laser is relatively inexpensive, this technique suffers from high effective process temperature and the quality of the resulting material is less than adequate.